Method of producing a magnetic material wire

ABSTRACT

A magnetic material wire is composed of a core of magnetic material having a Curie point of 70° to 250° C. and a high conductive metal sheathing of a uniform thickness covering the core. The ratio of the metal sheathing to the wire in cross-section is in the range of 15 to 40%. There is also disclosed a method of producing such a wire.

The application is a division of Ser. No. 506,013, filed on June 20,1983, now U.S. Pat. No. 4,525,432.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an elongated magnetic material element adaptedto be wound around a conductor of an overhead transmission line toprevent the freezing or icing thereof.

2. Prior Art

A conductor of an overhead transmission line is subjected to icing andthe deposition of snow in cold districts during the winter. The snow orthe ice on the conductor grows upon lapse of time to increase the weightof the conductor and a wind pressure to which the conductor issubjected, thereby excessively increasing a tension of the conductor,and a sag of the conductor between each adjacent pylons is undulyincreased. As a result, the conductor tends to be broken off, and thesteel pylons supporting the conductor tend to fall. Further, there is arisk that lumps of snow or ice drop from the conductor and hit apasser-by passing beneath the transmission line. Even if thetransmission line is laid over agricultural lands, such fallen lumps ofsnow or ice may give rise to damage to the crops and farm facilities.

In order to prevent the conductor from being subjected to the depositionof snow and the icing, it has been proposed to temporarily pass a largeamount of alternating current through the conductor to generate jouleheat by which the snow or ice on the conductor is melted. However, thismethod can not be carried out at all times because of the limitations onthe operation of the transmission line.

Another method of overcoming the above-mentioned difficulty is to mountrings on the conductor in spaced relation to cause the snow on theconductor to drop therefrom. However, the rings often fail to cause thesnow or the ice to drop satisfactorily. Further, lumps of the snow orice caused to drop by the rings may injure a passer-by or cause damageto the crops and the farm facilities.

It has also been proposed to mount a magnetic material element on thetransmission line conductor so that the snow or the ice on the conductoris melted by the heat due to hysteresis loss and eddy current lossgenerated by the magnetic field developing in the magnetic materialelement due to the flow of alternating current through the conductor.The magnetic material element includes a wire, a tape and a rod all ofwhich are adapted to be spirally wound around the conductor, and asleeve adapted to be fitted on the conductor. Such magnetic materialelement should be as lightweight as possible to prevent the transmissionline from becoming unduly heavy. Also, since the heat generated by themagnetic material element at temperatures causing no icing or snowdeposition contributes to the loss of the transmission power, themagnetic material element should preferably be made of a low Curie pointmaterial of which magnetic properties are lowered at high temperaturesto generate less heat. Generally, a low Curie point material tends to beless magnetic even at low temperatures than a high Curie point material.Therefore, the melting of snow or ice can not satisfactorily be achievedonly by the heat due to the hysteresis loss, and the heat due to theeddy current loss must also be used together to achieve a desiredmelting of the snow or ice.

Usually, the magnetic material element comprises a magnetic material anda conductive metal sheathing covering it.

In the case of a magnetic material having a high Curie point of not lessthan 300° C., the heat due to the hysteresis loss is greater than theheat due to the eddy current loss. Therefore, the melting effect is notso affected by the thickness of the conductive metal sheathing coveringthe magnetic material.

In the case of a magnetic material having a low Curie point of notlarger than 200° C., the heat due to the eddy current loss is greaterthan the heat due to the hysteresis loss. Therefore, it is necessary toproperly determine the thickness of the conductive metal sheathing inorder to achieve a desired melting of the snow or ice.

The magnetic material element has been made of alloys having a Curiepoint of 0° to 100° C., such as an alloy containing iron, nickel,chrominum and silicon and having a Curie point of around roomtemperature. However, magnetic properties of such alloys are liable tobe affected by heat treatment conditions and other processingconditions. In addition, such alloys have a poor reproducibility. Forexample, the magnetic material element in the form of a wire ismanufactured by drawing. Magnetic properties of the thus drawn wire arelowered due to the residual strain of the wire irrespective of thereduction rate of the drawing operation. If it is intended to use such awire for the purpose of melting the snow or ice on the conductor, alarge amount of wire must be wound around the conductor to achieve thedesired melting.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide a magneticmaterial element comprising a core of magnetic material having a lowCurie point and a high conductive metal sheathing of a uniform thicknesscovering the core, the thickness of the metal sheathing being sodetermined as to achieve the above-mentioned melting effect.

Another object is to provide such a magnetic material element having areduced residual strain.

According to the present invention, there is provided a magneticmaterial wire comprising a core of magnetic material having a Curiepoint of 70° to 250° C., and a high conductive metal sheathing of auniform thickness covering the core, the ratio of the metal sheathing tothe wire in cross-section being in the range of 15 to 40%.

If this ratio is less than 15%, the effect achieved by the conductivemetal sheathing is not satisfactory, and particularly when this ratio isnot less than 20%, a satisfactory effect is achieved. On the other hand,if this ratio exceeds 40%, the heat generated becomes unduly small. Mostpreferably, this ratio is 20 to 40%.

The core of magnetic material contains apart from impurities 32 to 52%by weight of nickel, 0.5 to 9% by weight of chromium, 0.2 to 2% byweight of silicon and balance iron. The core has a low Curie point of70° to 250° C.

The high conductive metal sheathing is made of copper, aluminum, zincand their alloys.

In the case of the cold-drawing of a metallic material, the residualstrain tends to develop in the material, and the amount of the strainbecomes greater toward the outer surface of the material because of thefrictional contact with the tools of the processing apparatus. It hasnow been found that this residual strain adversely affects the magneticproperties of a magnetic material. Generally, this strain can be reducedby a heat treatment. However, in the case of a magnetic material oralloy of the kind for the above application, its magnetic properties cannot sufficiently be recovered by a heat treatment. In addition, whensuch an alloy is heated at high temperatures, its strength is lowered,and a layer of intermetallic compound tends to be formed at theinterface between the magnetic material and the conductive metalsheathing. As a result, electrical conductivity and magnetic propertiesare adversely affected.

According to a further aspect of the present invention, the magneticmaterial element in the form of a wire is bent in such a manner that theratio of the radius (a half of the thickness) of the core to a radius ofcurvature of the bent wire is in the range of 2 to 9%, thereby reducingthe residual strain of the wire. With this method, the residual straincan be satisfactorily reduced without the need for a heat treatment.

The magnetic material wire according to this invention may have anycross-sectional shape such as oval, square and rectangular shapes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relation between the amount of heat per 1kg of an aluminum-sheathed wire and the ratio of the aluminum sheathingto the wire;

FIG. 2 is a graph showing the relation between the amount of heat per 1kg of a copper-sheathed wire and the ratio of the copper sheathing tothe wore;

FIG. 3 is a graph showing the relation between a saturated magnetic fluxdensity and a temperature;

FIG. 4 is a graph showing the relation between a hysteresis loss and atemperature;

FIG. 5 is a graph showing the relation between the amount of heat per 1kg of wire spirally wound on an ACSR and the magnetic field;

FIG. 6 is a triangular diagram showing the composition of alloyscontaining Fe, Ni, Cr and Si; and

FIG. 7 is a diagram similar to FIG. 6 but showing some examples of alloycompositions.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The invention will now be illustrated by the following examples:

EXAMPLE 1

36% by weight of nickel, 3% by weight of chromium, 1% by weight ofsilicon and balance iron (apart from impurities) were melted undervacuum and were cast under vacuum into an alloy ingot of 30 mm diameter.The alloy ingot was subjected to cold forging and was drawn into a wirecore of 10 mm diameter having a Curie point of around 150° C. Then, thesurface of the wire core was subjected to polishing. Then, 0.3 to 2.0 mmthick aluminum sheathings in the form of a tube were fitted respectivelyon a plurality of wire cores to form intermediate products. Then, theintermediate products were drawn into wires having diameters of 2.4 to3.0 mm, respectively, so that each of the wire core was reduced into adiameter of 2.3 mm. The ratio of the aluminum sheathing to the wire incross-section was 10 to 49%.

Comparative wires 1 and 2 were prepared according to the above procedureexcept that the aluminum sheathing was not applied to those wires, eachof the wire having a diameter of 2.3 mm. The comparative wire 2 wasgalvanized at a final stage of the manufacture.

Then, the wires of this invention and the comparative wires 1 and 2 wereplaced in an alternating magnetic field of 30 Oe (50 Hz), and the amountof heat generated by those wires was measured. The results obtained areshown in a graph of FIG.1.

Because of the limitation of the weight of the wire mounted on theconductor of the transmission line, the heat required for the melting ofsnow or ice on the conductor is at least 30 Watt per 1 kg of the wire inthe magnetic field of 30 Oe. As can be seen from the graph of FIG.1,when the ratio of the aluminum sheathing to the wire in cross-section is15 to 40%, the amount of heat generated is sufficient to achieve asatisfactory melting effect.

EXAMPLE 2

There was prepared a wire core made of an alloy composed of 36% byweight of nickel, 3.1% by weight of chromium, 1% by weight of siliconand balance iron, the wire core having a Curie point of around 130° C.

Another wire core was prepared from hard steel (JIS G 3506 SWRH 62 A).

Then, the alloy and steel cores were covered respectively withsheathings made of aluminum for an electrical application to producewire 1 of this invention and comparative wire 2a. In each case, theratio of the aluminum sheathing to the wire in cross-section was 25%.

Comparative wire 3 was prepared from the above-mentioned alloy and hadno sheathing. Also, comparative wire 4 was prepared from theabove-mentioned hard steel and had no sheathing.

Then, the wire 1 of this invention and the comparative wires 2a, 3 and 4were placed in an alternating magnetic field of 50 Oe and 15 Oe (50 Hz),and the amount of heat generated by those wires at a temperature of 0°C. was measured. The results obtained are shown in Table 1 in which theamount of heat is indicated in terms of Watt per 1 kg of each wire.

As can be seen from Table 1, the comparative wire 2a and the comparativewire 4 generated excessive heat. Thus, in the case where the wire ismade of hard steel regardless of whether it has a conductive metalsheathing, undue loss is produced.

The amount of heat generated by the wire 1 of this invention was aboutfour times as much as the amount of heat generated by the comparativewire 3. Thus, in the case where the magnetic material element is made ofan alloy containing iron and nickel, a conductive metal sheathing needto be provided.

                                      TABLE 1                                     __________________________________________________________________________                       Heat ratio  Heat ratio                                                        of sheathed of sheathed                                           Ratio of Al wire to non-                                                                              wire to non-                                          sheathing to                                                                         Watt/kg                                                                            sheathed wire                                                                        Watt/kg                                                                            sheathed wire                                         core   (50 Oe)                                                                            (50 Oe)                                                                              (15 Oe)                                                                            (15 Oe)                                        __________________________________________________________________________    Wire 1 of                                                                            25%    48   4.4    20   3.2                                            this invention                                                                Comparative                                                                           0%    11          6.2                                                 wire 3                                                                        Comparative                                                                          25%    105  1.3    9.8  0.93                                           wire 2a                                                                       Comparative                                                                           0%    82          10.5                                                wire 4                                                                        __________________________________________________________________________

EXAMPLE 3

Wire cores of an alloy composed of 54% by weight of nickel, 9% by weightof chromium, 0.5% by weight of silicon and balance iron were preparedaccording to the procedure in Example 1. Then, copper sheathings wereapplied to five wire cores so prepared so that intermediate productswere produced. Then, those five intermediate products were drawn to adiameter of 2 mm to produce wires 3 to 7 of this invention. The wireswere processed to reduce the residual strain thereof. The ratios of thecopper sheathing to the wire in cross-section in respect of the wires 3to 7 of this invention were 15%, 25%, 33%, 40% and 47%.

Comparative wire 5 was prepared according to the above procedure in thisExample except that the copper sheathing was not applied to the wire.The comparative wire 5 had a diameter of 2 mm and was processed toreduce the strain thereof.

Then, the wires 3 to 7 of this invention and the comparative wire 5 wereplaced in an alternating magnetic field of 30 Oe (50 Hz) with the axesof those wires disposed in the direction of the magnetic field, and theamount of heat generated by those wires was measured. The resultsobtained are shown in a graph of FIG. 2.

As can be seen from the graph of FIG. 2, in order to obtain a heatamount of at least 30 Watt per 1 kg of the wire, the ratio of the coppersheathing to the wire in cross-section need to be 15 to 40%.

EXAMPLE 4

1200 g of electrolytic iron of 99.9 weight % purity, 720 g of nickel of99.97 weight % purity, 60 g of chromium of 99.3 weight % purity and 20 gof metallic silicon of 98 weight % purity were melted in ahigh-frequency vacuum furnace to produce a molten material. Then, themolten material was cast into an ingot having a diameter of 30 mm and alength of 300 mm. Then, the ingot was subjected to hot forging attemperature of 1100° C. to form a wire core of 15 mm diameter. Then, thewire core was polished to remove oxide scales therefrom and subsequentlyreduced to a diameter of 6 mm by cold drawing. Then, the wire core wasfitted in an aluminum sheathing in the form of a tube to produce anintermediate product. Then, the intermediate product was processed orreduced to a diameter of 2.6 mm to produce a wire 8 of this invention ina manner not to affect the magnetic properties thereof.

EXAMPLE 5

432 g of electrolytic iron of 99.9 weight % purity, 259.2 g of nickel of99.97 weight % purity, 21.6 g of chromium of 99.3 weight % purity and7.2 of metallic silicon of 98 weight % purity were melted in ahigh-frequency vacuum furnace to produce a molten material. Then, themolten material was cast into an ingot having a diameter of 20 mm and alength of 300 mm. Then, the ingot was cold forged to a wire core of 10mm diameter. Then, the wire core was fitted in an aluminum sheathing inthe form of a tube to produce an intermediate product. Then, theintermediate product was drawn to a diameter of 2.6 mm in the samemanner described in Example 4, thereby producing a wire 9 of thisinvention.

EXAMPLE 6

385.2 g of electrolytic iron, 266.4 g of nickel, 64.8 g of chromium and3.6 g of silicon were melted in a high-frequency vacuum furnace to forman ingot. According to the procedure in Example 5, there was prepared awire 10 of this invention having a diameter of 2.6 mm and provided withan aluminum sheathing. The wire core of this wire has a low Curie pointof about 90° C., and has a saturated magnetic flux density of 4800 G anda hysteresis loss of 560 J/m³ at a temperature of 0° C. The amount ofthe heat generated by the wire 10 of this invention in an alternatingmagnetic field of 15 Oe (50 Hz) is 10 Watt per 1 kg of the wire, and theamount of heat generated in the alternating magnetic field of 30 Oe is16 Watt per 1 kg of the wire.

EXAMPLE 7

A wire core of 10 mm diameter was prepared according to the procedure inExample 5. Then, the wire core was fitted in a copper sheathing in theform of a tube to produce an intermediate product. Then, theintermediate product was drawn to a diameter of 2 mm to produce a wire11 of this invention in a manner not to affect the magnetic propertiesthereof. The wire core had the same composition as the wire cores inExamples 4 and 5 and hence exhibited the same magnetic properties. Inthe case where an aluminum-sheathed wire and a copper-sheathed wire havemagnetic material cores of the same composition, the amount of heatgenerated by the aluminum-sheathed wire is substantially the same as theamount of heat generated by the copper-sheathed wire, but since copperis heavier than aluminum, the copper-sheathed wire is less than thealuminum-sheathed wire in the amount of generation of heat per unitmass.

Also, for comparison purposes, comparative wires 6 and 7 were prepared.A hard steel wire (JIS G 3506-SWRH 57 B; analytical value by weight %:C--0.58, Si--0.25, Mn--0.80, P--0.02, S--0.01) having a diameter of 9.5mm was used as a wire core for the comparative wire 6. An aluminumsheathing was fitted on the hard steel wire to form an intermediateproduct. Then, the intermediate product was drawn to a diameter of 2.6mm to provide the comparative wire 6. The ratio of the aluminumsheathing to the wire in cross-section was 25%.

A soft steel wire (JIS G 3503-SWRY 11; analytical value by weight %:C--0.08, Si--0.02, Mn--0.50, P--0.01, Cu--0.05) having a diameter of 9.5mm was used as a wire core for the comparative wire 7. An aluminumsheathing was fitted on the soft steel wire to form an intermediateproduct. Then, the intermediate product was drawn to a diameter of 2.6mm to provide the comparative wire 7. The ratio of the aluminumsheathing to the wire in cross-section was 25%.

The wires 8 to 11 of this invention prepared respectively in Examples 4to 7 and the comparative wires 6 and 7 were placed in an alternatingmagnetic field of 25 Oe (50 Hz), and the relation between the saturatedmagnetic flux density (G) and the temperature (°C.) was observed. Theresults obtained are shown in a graph of FIG. 3. As can be seen fromthis graph, in respect of the wires 8 to 11 of this invention, thesaturated magnetic flux density becomes smaller with the increase of thetemperature. On the other hand, in respect of the comparative wires 6and 7, the saturated magnetic flux density becomes greater with theincrease of the temperature. Thus, the wires of this invention havesuperior properties.

Also, the relation between the temperature (°C.) and the hysteresis loss(J/m³) in respect of the wires 8 to 11 of this invention and thecomparative wires 6 and 7 placed in the magnetic field of 25 Oe wasobserved. The results obtained are shown in a graph of FIG. 4. As can beseen from this graph, in respect of the wires 8 to 11 of this invention,the hysteresis loss becomes smaller with the increase of thetemperature. On the other hand, in respect of the comparative wires 6and 7, the hysteresis loss does not become smaller with the increase ofthe temperature.

Also, the amount of heat, generated by each of the wires 8 to 11 andcomparative wires 6 and 7 wound at a pitch of 50 mm around a conductor(ACSR) through which current (50 Hz) flows to produce an alternatingmagnetic field in each wire, was observed. The conductor had aconductive cross-sectional area of 810 mm². The results obtained areshown in a graph of FIG. 5. As can be seen from this graph, when themagnetic field H is less than 20 Oe, the amount of heat generated by thewires 8 and 9 of this invention is greater than the amount of heatgenerated by the comparative wires 6 and 7, and in addition the wires ofthis invention have a smaller degree of increase of heat generation thanthe comparative wires.

Next, there was prepared a wire of 2.6 mm diameter consisting of analuminum sheathing and a wire core composed of 36% nickel, 3% chromium,1% silicon and balance iron (% by weight). This aluminum sheathed wirewas wound spirally around a conductor (ACSR), having a conductivecross-sectional area of 810 mm² and a diameter of 38.4 mm at a pitch of50 mm. Then, snow was caused to deposit on the ACRS with the spirallywound wire to carry out a snow-melting test under the conditions shownin Table 2. The results are also shown in Table 2.

                                      TABLE 2                                     __________________________________________________________________________              No wire                                                                       wound around                                                                  conductor                                                                             Test 1                                                                             Test 2                                                                              Test 3                                                                              Test 4                                     __________________________________________________________________________    Current (A)                                                                             300     300  300   200   200                                        Wind velocity                                                                            5      5.5  5.5   5.5   5.5                                        (m/s)                                                                         Snowfall (cm/H)                                                                          6.0    8.0  6.0   4.9   2.0                                        Temperature (°C.)                                                                -2.0    -2.0 -2.0  -2 to -2.5                                                                          -2 to -2.5                                           Ice-like                                                                              Sherbet-                                                                           Snow  Sherbet-                                                                            Sherbet-                                             snow    like melted                                                                              like  like                                                         snow completely                                                                          snow  snow                                       __________________________________________________________________________

As is clear from Table 2, when current of not less than 300 A flowsthrough the conductor, the snow deposited on the conductor with thealuminum-sheath wire is completely melted. When the current is 200 A,the deposited snow is not completely melted, but melted in asherbet-like manner, i.e., partly melted, so that the snow slides offthe conductor to achieve a substantial melting effect.

The magnetic material core of the wire of this invention contains, apartfrom impurities, 32 to 52% by weight of nickel (Ni), 0.5 to 9% by weightof chromium (Cr), 0.2 to 2% by weight of silicon (Si) and balance iron.When the nickel content is 32 to 52% by weight, the Curie point of themagnetic material core is lowered without deteriorating the magneticproperties, i.e., a saturated magnetic flux density and a hysteresisloss. When the nickel content does not fall within this range, asatisfactory effect can not be achieved. In addition, when the nickelcontent exceeds the upper limit of 52%, processability such as a drawingability is lowered.

The addition of 0.5 to 9% chromium serves to improve the magneticproperties to a satisfactory level and also to lower the Curie point.When the chromium content exceeds 9%, the magnetic properties aredeteriorated.

The addition of silicon achieves similar effects as the addition ofchromium, and its content should be not more than 2%. When its contentexceeds 2%, processability is adversely affected.

The above-mentioned composition range of the magnetic material core isindicated by a block in a triangular diagram of FIG. 6, the siliconcontent being 1%. Several examples of magnetic material cores arequalitatively indicated by A, B, C, D and E in a triangular diagram ofFIG. 7. Although the sample A has increased saturated magnetic fluxdensity and hysteresis loss, its Curie point is extremely high. Thesamples B and C are non-magnetic at a temperature of around 0° C.Although the sample D has increased saturated magnetic flux density andhysteresis loss, its processability is lowered. The sample E which fallswithin the range of this invention has increased saturated magnetic fluxdensity and hysteresis loss at a temperature of around 0° C.

EXAMPLE 8

Wire cores a, b, c and d having respective compositions shown in Table 3were prepared by casting ingots of 30 mm diameter by the use of a vacuumfurnace and then by reducing the ingots to a diameter of 10 mm by hotforging and cold forging. Then, the wire cores were cleaned by removingscales of oxides and oil from their surfaces. Then, sluminum sheathingsin the form of a tube having an outer diameter of 12 mm and a thicknessof 0.8 mm were fitted on the wire cores a, b, c, and d to produceintermediate products. Then, the intermediate products were drawn to adiameter of 2.6 mm to produce wires a', b', c' and d' having the wirecore a, b, c, and d, respectively, the aluminum sheathing and wire coreof each wire being metallically bonded together. The sulfur andphosphorus contents of each wire core are impurities.

The wires a', b', c' and d' were bent to apply a reverse strain to theentire outer peripheral portion of the wire core to reduce the residualstrain inherent in the wire core. The ratio of the radius r of the wirecore to a radius R of curvature of the bent wire was 1.5%, 5.2% and9.5%. The wires a', b', c' and d', subjected to this bending operationand the wires a', b', c', and d' not subjected to such bending wereobserved in respect of the magnetic properties, using a DC magnetizationmeasuring device. The results obtained are shown in Table 4. As seenfrom Table 4, the wires subjected to the bending operation exhibitedmuch improved magnetic properties.

                  TABLE 3                                                         ______________________________________                                         (Wt. %)                                                                      Ni        Cr       Si    S      P    Fe                                       ______________________________________                                        a      36.2   3.1      1.0 0.005  0.003                                                                              Balance                                b      37.3   8.9      0.5 0.004  0.003                                                                              "                                      c      36.5   9.0      1.0 0.005  0.004                                                                              "                                      d      45.7   5.1      1.1 0.006  0.004                                                                              "                                      ______________________________________                                    

                                      TABLE 4                                     __________________________________________________________________________    No bending operation                                                                        T/R: 5.2% r/R: 1.5%   r/R: 9.5%                                 Bs (Gauss)                                                                            Wh (J/m.sup.3)                                                                      Bs (Gauss)                                                                          (J/m.sup.3)                                                                       Bs (Gauss)                                                                          Wh (J/m.sup.3)                                                                      BS (Gauss)                                                                          Wh (J/m.sup.3)                      __________________________________________________________________________    a'                                                                              6,480 1,055 9,260 1,210                                                                             6,520 1,070 mechanically                              b'                                                                              5,780   719 7,480   738                                                                             5,840   725 damaged                                   c'                                                                              5,210   512 7,250   681                                                                             5,280   530                                           d'                                                                              8,720 1,384 12,300                                                                              1,648                                                                             8,850 1,420                                           __________________________________________________________________________     (Temperature: 0° C; Magnetic field: 30 Oe)                        

EXAMPLE 9

According to the procedure in Example 8, there was prepared analuminum-sheathed wire of 2.6 mm diameter having a wire core containingapart from impurities 62% by weight of nickel, 3.1% by weight ofchromium, 1% by weight of silicon and balance iron. Seven samples 1 to 7were prepared from the wire so formed and bent under the conditionsshown in Table 5 so that a bending strain (r/R) was applied to the outerperipheral portion of the wire. The magnetic properties of those sampleswere observed according to the procedure in Example 8.

Samples 3 to 6 having a bending strain in the range of 2 to 9% exhibitedmuch improved magnetic properties. Sample 2 having a bending strain ofless than 2% was not significantly improved in the magnetic properties.When the bending strain exceeds 9%, the wire is subjected to amechanical damage such as meandering and rupture.

                  TABLE 5                                                         ______________________________________                                                 Bending strain (%)                                                   Sample No.                                                                             (r/R)         Bs (Gauss) Wh (J/m.sup.3)                              ______________________________________                                        1        0             6,480      1,055                                       2        1.5           6,520      1,070                                       3        2.3           8,970      1,120                                       4        5.2           9,260      1,210                                       5        6.7           9,360      1,220                                       6        8.3           9,540      1,240                                       7        9.5           mechanically damaged                                   ______________________________________                                    

EXAMPLE 10

Three kinds of aluminum-sheathed wires a", b" and e of 2.6 mm diameterwere prepared using the wire cores a and b in Table 3 and a wire core ofcarbon steel, respectively. Samples were prepared from those wires andsubjected to a bending operation under the conditions shown in Table 6.The amount of heat generated by each sample in an alternating magneticfield of 15 Oe (50 Hz) at a temperature of 0° C. was measured. Theresults obtained are shown in Table 6. The heat amount is indicated interms of Watt per 1 kg of each sample.

For comparison purposes, the heat amount of the samples not subjected tothe bending operation was also measured.

As seen from Table 6, the samples a" and b", having the respective coresa and b and subjected to the bending operation to reduce the residualstrain, generated a much larger amount of heat than those not subjectedto the bending operation. On the other hand, the sample e having thewire core of carbon steel was not improved in the generation of heateven though it was subjected to the bending operation. Thus, only in thecase of the magnetic material wire having a core of an iron-nickel basedalloy, the reduction of the strain is efficient in improvement of themagnetic properties and heat generation.

As described above, when the bending strain (r/R) is less than 2%, adesired heat amount is not achieved. Also, if the bending strain is morethan 9%, the wire is mechanically damaged.

Therefore, the bending strain, i.e., the ratio of the radius r of thewire core to the radius R of curvature of the bent wire should be 2 to9% in order to achieve the desired heat amount for melting the snowwithout sacrifice of the mechanical strength of the magnetic materialwire.

                  TABLE 6                                                         ______________________________________                                                           Bending                                                    Sample             strain (r/R)                                                                            Watt/kg                                          ______________________________________                                        a"     (strain reduced)                                                                              5.2%      22.3                                         b"     (strain reduced)                                                                              5.2%      17.6                                         e      (strain reduced)                                                                              5.2%       9.6                                         a"     (no strain removed)                                                                           --        10.2                                         b"     (no strain removed)                                                                           --         8.3                                         e      (no strain removed)                                                                           --         9.5                                         a"     (strain reduced)                                                                              1.5%      12.2                                         b"     (strain reduced)                                                                              1.5%      10.4                                         ______________________________________                                    

What is claimed is:
 1. A method of producing a magnetic material wireadapted to be wound around an electric conductor for generating heatwhich comprises the steps of:(a) preparing an elongated core of amagnetic material having a Curie point of 70° to 250° C., said corecontaining apart from impurities 32 to 52% by weight of nickel, not lessthan 0.5 but less than 9% by weight of chromium, 0.2 to 2% by weight ofsilicon and balance iron; (b) covering said core with a high conductivemetal sheathing of a uniform thickness to form an intermediate product;(c) reducing said intermediate product to a predetermined cross-sectionto form the wire, the ratio of the metal sheathing to the wire incross-sectional area being in the range of 15 to 40%; and (d) bendingthe wire to apply a reverse strain to the entire peripheral portion ofsaid core in such a manner that the ratio of the radius of the core tothe radius of the curvature of the bent wire is in the range of 2 to 9%,thereby reducing the residual strain of the wire so that the magneticproperty of said core contributable to the heat generation of the wireis improved.
 2. The method of producing a magnetic material wireaccording to claim 1, wherein said magnetic metal sheathing is made of amaterial selected from the group consisting of copper, aluminum, zincand their alloy.